1. Field of the Invention
The present invention relates to a cellular system, a base station, a mobile station and a communication control method therefor, and particularly to a cellular system, a base station, a mobile station and a communication control method therefor that can increase reliability of transmission of control information in High-Speed Downlink Packet Access (HSDPA).
2. Description of the Related Art
The HSDPA for transmitting high-speed data over a downlink line from a base station to a mobile station in a cellular system is studied in the 3GPP (3rd Generation Partnership Project). In this HSDPA, the High-Speed Physical Downlink Shared Channel (HS-PDSCH) is used for transmission over the downlink line from the base station to the mobile station. This HS-PDSCH is used for sending data from each base station to a plurality of mobile stations. Therefore, the base station or a controller thereof determines a schedule for sending data to each of the plurality of mobile stations, and sends data in different timing for each mobile station.
For controlling transmission of data from the base station to the mobile station as described above, each base station sets a DPCH (Dedicated Physical Channel) individually between itself and each of the plurality of mobile stations. This DPCH is used for sending control information from the base station to the mobile station by its downlink signal, and sending control information from the mobile station to the base station by an uplink signal in the reverse direction. Control information that is sent from the base station to the mobile station includes, for example, information of timing for sending data to the mobile station. In addition, for the HS-PDSCH, there exists a technique in which a modulation system enabling the highest-speed data transmission to be carried out within the bounds of achieving an intended bit error rate is selected to be used from a plurality of modulation systems (e.g. QPSK, 16QAM, 64QAM) depending on the state of a transmission line between the base station and the mobile station, with its transmission power fixed, but information of selection of the modulation system is also sent from the base station to the mobile station as control information.
On the other hand, control information that is sent from the mobile station to the base station includes, for example, reception confirmation notification information for each data block in the case where data is divided into a plurality of blocks, and is sent from the base station to the mobile station. There are also cases where for changing a modulation mode, quality of reception of common pilot signal sent from the base station is determined, and the result of the determination is sent from the base station to the mobile station as control information.
In each mobile station, the percentage of time during which data is received using the HS-PDSCH is small, but even in the state of standby for receiving data in which no data is being received, the DPCH continues to be set between the mobile station and the base station, thus making it possible to start transmission of data in a short time when the transmission of data is requested. Therefore, the number of mobile stations to which each base station sends data at a time is only one, but a large number of mobile stations are in the state of standby, and set the DPCH between themselves and the base station.
In the cellular system, there exists a technique called a soft handover in which the mobile station sets channels with a plurality of base stations at a time. Each base station sends the common pilot signal with predetermined power, and the mobile station sets the DPCH with a base station of which power for reception of the common pilot signal is the largest, but in the soft handover, when a different base station that is not so much different in power for reception of the common pilot signal exists, the mobile station sets the DPCH with the different base station to set the DPCH with a plurality of base stations. Hereinafter, the base station with which the DPCH is set in this manner is referred to as a connection base station.
In addition, in the cellular system, a technique called transmission power control of high-speed closed loop type may be applied. The transmission power control of high-speed closed loop type is applied to one or both of the uplink line and downlink line for the DPCH. For the transmission power control for the uplink line of the DPCH, the base station uses a dedicated pilot signal included in the uplink signal to determine its reception SIR (Signal to Interference Ratio), and compares the determined value with a predetermined desired SIR. Then, if the determined value is smaller than the desired SIR, the base station includes TPC (Transmit Power Control) bits indicating growth of power in the downlink signal of the DPCH to make a notification to the mobile station, and if it is not the case, the base station includes TPC bits indicating reduction of power in the downlink signal of the DPCH to communicate the same to the mobile station. Then, the mobile station receives the TPC bit, and increases or decreases the transmission power in accordance with the TPC bit. In the case where this transmission power control of the uplink line is used in combination with the soft handover, the mobile station receives the TPC bit from each of a plurality of connection base stations, and when at least one TPC bit indicates reduction of power, the mobile station decreases the transmission power of the DPCH, and if it is not the case (specifically, if all TPC bits indicate growth of power), it increases the transmission power of the DPCH. By carrying out such transmission power control, a desired SIR is achieved as quality of the uplink line signal in at least one connection base station, and the quality of reception of the uplink line signal is prevented from exceeding the desired SIR in all the connection base stations, thereby preventing interference wave power of the uplink line from being increased.
On the other hand, for transmission power control of the downlink line of the DPCH, the mobile station uses a dedicated pilot signal included in the downlink signal to determine its reception SIR, and compares the determined value with a predetermined desired SIR. Then, if the determined value is smaller than the desired SIR, the mobile station includes TPC bits indicating growth of power in the uplink signal of the DPCH to make a notification to the base station, and if it is not the case, the mobile station includes TPC bits indicating reduction of power in the uplink signal of the DPCH to communicate the same to the base station. Then, the base station receives the TPC bits, and increases or decreases the transmission power in accordance with the TPC bit. In the case where this transmission power control of the downlink line is used in combination with the soft handover, the mobile station receives the downlink line signal of the DPCH from each of plurality of connection base stations and synthesizes the same, and compares the synthesized reception SIR of the downlink line signal with the desired SIR to determine TPC bits. Then, common TPC bits are sent to a plurality of connection base stations, and each of connection base stations increases or decreases transmission power in accordance with the TPC bits. In this way, all connection base stations increase or decrease transmission power in accordance with the common TPC bits, thereby maintaining the balance among connection base stations, and the downlink line signal sent by the connection base station for which loss in transmission between itself and the mobile station is the smallest is received in good quality by the mobile station, so that the transmission power of the downlink line signal is prevented from being increased more than necessary to prevent the interference wave power of the downlink line from being increased.
The transmission power control and soft handover described above are effective techniques for reducing the interference wave power to increase line capacity by reducing the transmission power as a radio access method, particularly in a cellular system based on The CDMA (Code Division Multiple Access) method.
There are cases where a technique called a FCS (Fast Cell Selection) is applied for HS-PDSCH. This FCS is used in combination with the soft handover. In the FCS, data to be transmitted to the mobile station is sent to each of connection base stations. Then, the mobile station determines power for reception of the common pilot signal sent from each of the connection base stations, and communicates to each of the connection base stations the identification code of a connection base station of which reception power is the largest (hereinafter referred to as Primary base station). On the other hand, each of the connection base stations sends data by the HS-PDSCH if the communicated identification code is identical to that of its own station, and does not send data by the HS-PDSCH if it is not the case. The connection base station sending data in this way is frequently changed, whereby a connection base station with the transmission line in the best condition sends data, thus making it possible to transmit data more speedily when one modulation system is selected from a plurality of modulation systems with the transmission power fixed. In this FCS, information of the identification code the mobile station communicates to the connection base station is also control information that is sent from the mobile station to the base station by the uplink line signal of the DPCH for data transmission by the HS-PDSCH.
Furthermore, a technique called SSDT (Site Selection Diversity Transmit Power Control) may be applied for the DPCH. This SSDT is a technique similar to the FCS, and is used in combination with the soft handover. In the SSDT, the mobile station communicates the identification code of the Primary base station to each connection base station, and each connection base station sends the downlink line signal of the DPCH if the communicated identification code is identical to that of its own station, and does not send the downlink line signal of the DPCH if it is not the case. The connection base station sending the downlink line signal of the DPCH in this way is frequently changed, whereby a connection base station with the transmission line in the best condition sends data, and therefore when the transmission power of the downlink line signal is controlled so that the reception SIR of the downlink line signal in the mobile station tales on a predetermined desired value, the transmission power of the downlink line signal of the DPCH can be minimized, thus making it possible to increase line capacity. This SSDT is disclosed in Japanese Patent No. 2991185 and Japanese Patent No. 3047393.
For data transmission using the HS-PDSCH described above, if reliability of control information by the DPCH is low, control information reception errors in the base station and mobile station are increased, and thus efficiency of data transmission is reduced. The HS-PDSCH has larger transmission power than the downlink signal of each DPCH because high-speed data transmission is carried out, and therefore if transmission of a data block is failed and then the block is sent again, the interference wave power of the downlink line is significantly increased to reduce the line capacity.
As a method for preventing such reduction of efficiency of data transmission, a method can be considered in which the desired SIR used as the value of reception SIR is set to be a large value in the transmission power control of high-speed closed loop type. In the transmission power control of the uplink line, the base station sets the desired SIR to be a large value, whereby the mobile station sends the uplink line signal of the DPCH with large power, and the reception SIR of the uplink line signal received by the base station is increased, thus increasing reliability of control information. On the other hand, in the transmission power control of the downlink line, the mobile station sets the desired SIR to be a large value, whereby the base station sends the downlink line signal of the DPCH with large power, and the reception SIR of the downlink line signal received by the mobile station is increased, thus increasing reliability of control information.
In addition, in the FCS described above, each of the connection base stations makes a judgment on the identification code of the Primary base station communicated from the mobile station, and determines whether or not data is sent, depending on the result of the judgment. Therefore, reliability of control information is low, and if the Primary base station makes an erroneous judgment on the identification code, data is not sent, thus reducing efficiency of data transmission. In addition, if a connection base station other than the Primary base station makes an erroneous judgment on the identification code to send data, unnecessary data is sent, and thus the interference wave power is increased and the line capacity is decreased.
Because a desired SIR is achieved as quality of reception of the uplink line signal in at least one connection base station as described above, it is likely that the quality of reception of the uplink line signal in other connection base stations is below the desired SIR, if this FCS is used in combination with the transmission power control of the uplink line. Since the identification code of the Primary base station is included as control information in this uplink line signal, reliability of the identification code is high in at least one connection base station, but reliability of the identification code is low in other connection base stations. For a system in which the frequency used in the uplink line is different from the frequency used in the downlink line, because phasing for the uplink line is different from that for the downlink line, and the propagation loss in the uplink line of the Primary base station is not necessarily a minimum, reliability of the identification code in the Primary base station may be decreased. Thus, if it is used in combination with the transmission power control of the uplink line, in particular, the probability of occurrence of erroneous judgment on the identification code is high, and therefore efficiency of data transmission is reduced and the line capacity is decreased.
As a measure against the above problem, the connection base station sets the desired SIR to be a large value in the transmission power control of the uplink line, whereby reliability of the identification code can be increased in a larger number of connection base stations, thus making it possible to prevent a situation in which the efficiency of data transmission is reduced and the line capacity is decreased.
In addition, as another measure, a method in which the transmission power control of the uplink line during soft handover is carried out in a way different from the aforesaid method, namely a method in which when receiving TPC bits from each of a plurality of connection base stations, the mobile station increases the transmission power of the DPCH if at least one TPC bit indicates growth of power, and the mobile station decreases the transmission power of the DPCH if it is not the case (specifically if all TPC bits indicate reduction of power) is now under review in the 3GPP. In addition, a method in which when TPC bits are received from the current Primary base station or a new Primary base station, the transmission power of the mobile station is increased if at least one TPC bit indicates growth of power is also under review in the 3GPP. With these methods, the quality of reception of the uplink line signal can be brought close to the desired SIR to increase reliability of the identification signal in all the connection base stations and the Primary base station, thus making it possible to prevent a situation in which the efficiency of data transmission is reduced and the line capacity is decreased.
In addition, in the SSDT described above, if in connection base stations, the Primary base station erroneously receives the identification code of the Primary base station communicated from the mobile station to send no downlink line signal of the DPCH, none of the connection base stations sends the downlink line signal of the DPCH, and consequently reliability of the control information is decreased, efficiency of data transmission in the HS-PDSCH is reduced and the line capacity is decreased. As a measure against the above problem, the method not applying SSDT to DPCH is considered.
In addition, there are cases where the base station uses an adaptive antenna technique in which a dedicated pilot signal included in the uplink signal is used to estimate the arrival report of the uplink signal, an antenna directional pattern is adaptively formed so that the directional gain for the arrival direction is increased, and data is sent from the base station to the mobile station using the HS-PDSCH. The dedicated pilot signal consists of a predetermined code system, which is used as a reference signal to estimate the arrival direction. This adaptive antenna technique is described in detail in “Adaptive Array and Mobile Communication” (II) (The Institute of Electronics, Information and Communication Engineers Journal, Vol. 82, No. 1, pp. 55-61, January, 1999) and “Adaptive Array and Mobile Communication” (IV) (The Institute of Electronics, Information and Communication Engineers Journal, Vol. 82, No. 3, pp. 264-271, March, 1999), etc.
Due to this adaptive antenna technique, the power for reception of the HS-PDSCH is large in a location where the mobile station exists, and the reception power is relatively small in other locations, and therefore satisfactory line quality is obtained while reducing the interference wave power for other cells. At this time, if a modulation system enabling data to be transmitted at a maximum speed is selected from a plurality of modulation systems, and is used, higher-speed data transmission can be carried out even if transmission power is fixed. In addition, interference wave power for other cells is decreased, and the line capacity is increased.
In this adaptive antenna technique, the dedicated pilot signal needs to be received in good quality for improving accuracy of estimation of the arrival direction. As a measure for achieving this purpose, a method in which the base station sets the desired SIR to be a large value in the transmission power control of the uplink line can be considered.
However, in the transmission power control of the uplink line, if the desired SIR for use as a desired value of the reception SIR of the uplink line signal in the base station is set to be a large value, the transmission power of the uplink line signal is increased. Similarly, in the transmission power control of the downlink line, if the desired SIR for use as a desired value of the reception SIR of the downlink line signal in the mobile station is set to be a large value, the transmission power of the downlink line signal is increased. Also, in the transmission power control of the uplink line during soft handover, in the case where control is carried out for bringing the quality of reception of the uplink line signal close to the desired SIR in all the connection base stations or the Primary base station, the transmission power of the uplink signal is increased. As described previously, there is a disadvantage that since a large number of mobile stations are in the state of standby in which no data is being received using the HS-PDSCH, and set the DPCH between themselves and the base station, the interference wave power is increased and the line capacity is decreased as their transmission power is increased.
The transmission power is further increased due to an increase in interference wave power resulting from an increase in transmission power of the signal in other DPCHs. This is because for the signals of the uplink and downlink lines of the DPCH, the transmission power is controlled so that their reception SIRs are brought close to predetermined desired values. In this way, among a plurality of DPCHs, the transmission power is increased exponentially as the number of DPCHs set at a time is increased because they interfere with one another. As described previously, there is a disadvantage that since a large number of mobile stations are in the state of standby in which no data is being received using the HS-PDSCH, and set the DPCH between themselves and the base station, the interference wave power is increased exponentially, and the line capacity is significantly decreased if the transmission power of the uplink and downlink line signals of a large number of DPCHs is increased.
In addition, for a method in which the SSDT is not applied to the DPCH so as to prevent reliability of control information from being decreased due to the SSDT, there is a disadvantage that a large number of mobile stations are in the state of standby, and thus set the DPCH between themselves and the base station, but the SSDT cannot be applied to the DPCH of a large number of mobile stations, and therefore an effect of reduction of transmission power of the downlink line signal from the SSDT cannot be obtained, thus increasing the interference wave power in the downlink line, and decreasing the line capacity.